Magnetic roller for electro-photographic development



July 16, 1968 G. NAUMANN 3,392,432

MAGNETIC ROLLER FOR ELECTROPHOTOGRAPHIC DEVELOPMENT Filed Dec. 16, 1964INVENTOR.

' GERHARD NAUMANN FIG 5 r J 0, BY ATTORNEY.

United States Patent Oflice 3,392,432 Patented July 16, 1968 3,392,432MAGNETIC ROLLER FOR ELECTRO- PHOTOGRAPHIC DEVELOPMENT Gerhard Naumann,Wiesbaden, Germany, assignor, by mesne assignments, to AzoplateCorporation, Murray Hill, NJ.

Filed Dec. 16, 1964, Ser. No. 418,606 Claims priority, applicationGermany, Dec. 18, 1963, K 51,671 9 Claims. (Cl. 29-110) The inventionrelates to a device for the development of latent electrostatic images,in which a mixture of magnetizable carrier particles and toner powder isapplied to a surface carrying a latent image by means of a rollercomprising a permanent magnet exciter system and a roller shellsurrounding the exciter system. The magnetic excitation of the rollershell can, as desired, be fixed or variable between zero and a maximumvalue.

Magnetic rollers are known which have, parallel to their axes, a seriesof permanent magnet or electricallyexcited magnetic poles and which dipinto a storage container containing developer mixture and attract thelatter magnetically. By rotation of the roller, the mixture taken up byit is applied to the surface to be developed. In the case of such knownmagnetic rollers, it is diflicult to compensate for the impoverishmentin toner powder of the developer mixture during the developing process,since without complicated additional devices susceptible to mechanicaltrouble, e.g. metal scrapers, the same magnetized carrier particlesalways adhere to the magnetic roller, as a result of which no sufficientmixture of the entire developer supply and enrichment of the carrierparticles with toner can be effected.

This disadvantage can be avoided with electrically excited magneticrollers since, in the case of such rollers, there exists the possibilityto periodically switch off the total magnetization during or between twodeveloping processes, or periodically to switch off the magnetization ofindividual magnetic poles during a part of their rotation. The use ofelectrically magnetized rollers is, however, also accompanied bydisadvantages. The manufacture of the rollers is complicated andexpensive which, especially in the case of the usual long magneticrollers, is important. The magnetizing current must be supplied throughslip rings or a collector; this can very easily give rise to trouble.The ohmic losses in the magnet windings cause, in the case of operationover lengthy periods, a sometimes very detrimental heating resulting inagglomeration of the developer mixture. Finally, for the operation ofelectrically magnetized rollers, they must have a power supply unit oftheir own, the space occupied by which and the heat developed by whichare obstacles to the desirable compact construction of developingdevices.

The present invention provides a magnetic roller which combines theabove-mentioned advantages of permanent and electrical magnetizationwithout the disadvantages associated therewith.

The present invention more specifically relates to a magnetic roller forthe development of latent electrostatic images with a developer mixturewhich contains toner powder and magnetizable particles, comprising aroller shell having substantially axially parallel so'ft magnetic poleshoes distributed over the circumference thereof and being separatedfrom one another by non-magnetic separatin g strips: a magnetizingsystem inside the roller shell comprising substantially axially parallelpermanent magnet poles distributed over the circumference of the roller,

the magnetizing system and the roller shell being capable of rotatinground a common axis.

The device according to the invention provides in an advantageous mannerthe possibility to switch off the magnetization of the developer roller.Since the magnetization takes place by means of a magnetizing systemcontaining permanent magnets, all the electrical devices whicharerequired in the operation of a conventional electromagnetic rollercapable of being switched oil are thus unnecessary. This results in asubstantially reduced susceptibility to trouble and a simple devicerequiring a relatively small amount of space.

Compared with the known, permanent magnet rollers which cannot beswitched off there is achieved in desirable manner the possibility toswitch off over the entire circumference of the roller or over anydesired portion thereof. In addition, there is provided a variation ofthe strength of the magnetic field and thus an influencing of the brushhardness of the developer powder to be taken up, i.e., practically aninfluencing of the amount of powder attracted.

The magnetic roller according to the invention comprises two parts, aroller shell and a magnetizing system. The roller shell is tubular andpossesses a common axis with the cylindrical magnetizing system, aroundwhich common axis the two systems are capable of rotating. The rollershell and magnetizing system together form the developer roller of theinvention.

The roller shell is divided into individual pole shoes which aredistributed in a substantially axially parallel manner over the entirecircumference of the shell. The number of the pole shoes is not criticalbut an even number is preferable. The pole shoes are separated from oneanother by non-magnetic separating strips so that the tube-shaped rollershell is alternately made up of pole shoe-separating strip-poleshoe-separating strip, etc. The extension of the pole shoes in a radialdirection is also not critical. This will be dependent on the design ofthe magnetizing system which is inside the roller shell and on theroller diameter required.

The pole shoes consist of soft magnetic material which is magnetizedonly for the duration of the eliect of a magnetic field and which, afterthe collapse of the magnetic field, retains only a small residualmagnetism. Materials which are suitable are, for example, soft iron,cobalt, nickel or the known Heusler alloys.

The separating strips preferably are made of a material having a lowmagnetic permeability, suitable examples being numerous metals, such asaluminum, brass and others, as well as thermoplastic and thermosettingplastics. The separating strips may, however, also be air slits. In thiscase, the penetrations of magnetizable particles of the developer intothe slits must, of course, be prevented, since otherwise these particleswould nullify the separating effect of the strips.

The combination of the magnetic separating strips and the pole shoesinto a rigid complete tube can be alfected by mechanical connectingmembers at the roller ends such as tension hoops etc., or by bonding orby end flanges. These constructional elements preferably consist ofnon-magnetic material so that they do not influence the fielddistribution of the magnetic roller at the roller ends. Because of theresidual magnetism of the pole shoe material, which magnetism ispractically always present, it has proved advantageous to provide theouter surface of the roller shell with a thin, smooth covering of aplastic, for example a known thermosetting molding composition, or witha thin non-magnetizable metal foil, in order to promote thedisengagement of the developer powder during the demagnetized state ofthe roller shell.

The cylindrical magnetizing system inside the tubular roller shellincludes one or several permanent magnets which are so magnetized that,distributed over the cirmumference, axially parallel magnetic polespoint outwardly to the roller shell. When several permanent magnets areused, they may be mounted on a soft magnetic core. Suitable metallicpermanent magnet materials are, for example, alloys of iron-cobaltvanadium, coppernickel-iron, and copper-nickel-cobalt. Exemplary ofoxidic permanent magnet materials are barium and cobalt ferrites.

The number of magnet poles facing the roller shell should at the most beequal to the number of the pole shoes; an equal number is preferred.Between the individual magnet poles there are intermediate spaces whichpossess a high magnetic resistance, i.e., they consist of anon-magnetizable material.

The magnetizing system and the surrounding shell rotate around a commonaxis. Magnetizing system and shell can be adjusted relative to eachother in such a manner that the magnetizing system can be rotated aroundthe common axis to take different positions with regard to the shell.Preferably, the magnetizing system is automatically displaced withregard to the shell at the beginning of the common revolution, andturned back to its original position at the end of said commonrevolution, this being effected, e.g., by providing a drive for only oneof these parts, whereas the other is a braked part and entrained by thedriven part by means of an adjustable catch against the action of aspring, the angle of rotation or shift of the magnetizing systemrelative to the shell being defined by stationary or adjustably fixedstops. If the drive is switched on, the driven part begins to revolve,and only when the catch reaches the stop mounted in front thereof, thebraked part is taken along. When the drive motor is switched off, therotational shift of the two parts relative to each other is revokedbecause the spring displaces the two parts in relation to each otheruntil the catch reaches the second stop mounted behind the catch.Alternatively, the two parts may be driven separately. This may beachieved by known mechanical means, e.g., by a combination of separatelydriven knockout spindles. If, as a result of a particular adjustment ofthe magnetizing system in relation to the roller shell, one magnet poleof the magnetized system lies exactly underneath one pole shoe of theroller shell, this magnet pole produces a magnetic induction in the poleshoe whereby a magnetic field forms which extends beyond the rollersurface. This renders it possible to attract the magnetizable particlesof the developer mixture. These form a so-called magnetic brush by whichthe developer powder can, in conventional manner, be brought to thesurface of the latent electrostatic image to be developed. If themagnetizing system is now rotated in relation to the roller shell, and,in an extreme case, so far that the magnet poles of the magnetizingsystem are exactly opposite the non-magnetic separating strips of theroller shell, there occurs only an infinitesimal magnetic field outsidethe roller shell, since practically all field lines going out from themagnet poles go inside the pole shoes to the adjacent magnet poles. Inthis position of the magnetizing system with respect to the rollershell, no developer mixture is therefore attracted.

Between the two extreme cases mentioned, namely the maximum formationand the practically complete disappearance of the outer magnetic field,all intermediate strengths are realizable. This can take place byrotation of the magnetizing system with respect to the roller shell byangles which are smaller than the angle between the two extremepositions. The strength of the magnetic field, and thus the strength ofthe attraction of the developer mixure and the hardness of the magneticbrush, can

therefore be adjusted continuously or discontinuously up to a maximum.

The width of the non-magnetic separating strips between the pole shoesof the roller shell may vary within wide limits. By the width, theamount of attracted developer mixture, its distribution around thecircumference of the roller shell, and thus the hardness of the magneticbrush, can be influenced. The width of the separating strips must,however, be large enough for their magnetic resistance to be greaterthan the magnetic resistance of the developer mixture held fast betweentwo pole shoes. Since the mixture contains a high proportion of aferromagnetic material, this condition is fulfilled where the width ofthe separating strips is only a little greater than the diameter of theferromagnetic particles contained in the developer mixture.

When the width of the separating strips is relatively small, themagnetic lines of force emerge from the pole shoes primarily in theareas adjacent to the separating strips, so that the field strength inthese areas is substantially greater than in the middle of the poleshoe. In this case, therefore, there is formed a relatively hardmagnetic brush whose density varies somewhat around the circumference ofthe roller. If, on the other hand, the width of the separating strips isrelatively large, the field strength differences between the middle ofthe pole shoes and the areas of the pole shoes adjoining the separatingstrips are very much smaller so that a softer magnetic brush withpractically uniform density around the roller circumference is formed.

The separating strips between the pole shoes may extend in a radialdirection or in a direction inclined thereto. In the first case, themagnetic brush formed by the roller is in mirror symmetry with referenceto every plane containing the rotational axis of the magnetic roller andthe middle of the pole shoes or the middle of the separating strips. Inthe second case, this symmetry is not present, since then the fieldstrength of the pole shoe at the two separating strips adjacent to eachpole shoe is different. As a result, the stiffness or hardness of themagnetic brush held by such a magnetic roller is unsymmetrical, forexample greater in the direction of rotation than in the oppositedirection. This fact can be favorable for special developing problems.

The above-described effect which the separating strips have on themagnetic field is governed primarily by the Width and form of theseparating strips at the outer surface of the roller shell, whereas thewidth and form of the separating strips at the inner surface of theshell have a substantially smaller effect on the form and hardness ofthe magnetic brush. The cross-section of the separating strips can,therefore, be chosen substantially according to manufacturingconsiderations. The sole requirement is that the magnetic resistance ofthe separating strips over their entire radial extension should beappreciably greater than the magnetic resistance of the part of themagnetic circuit-magnetizing system-pole shoe-developer mixturepoleshoe-magnetizing system consisting of two adjacent pole shoes and thedeveloper mixture held fast thereby.

Referring to the accompanying drawings:

FIGURES 1 and 2 show a cross-section of one example of an embodiment ofthe magnetic roller of the invention in which the magnetization as awhole can be switched on and off,

FIGURE 3 is a further embodiment of the magnetic roller of the inventionin which the magnetization can be switched on and off and, in addition,the magnetization of the pole shoes can be interrupted periodically in afixed angular region, and

FIGURES 4 and 5 show particular examples for the formation of themagnetizing system.

In FIGURE 1, the magnetic roller comprises an even number of pole shoes1-10 of soft iron which, in each case, are separated from one another bythe separating strips 2-2c of non-magnetic material and which are unitedinto a tube by conventional means not shown, e.g. end

flanges. The magnetizing system inside the tube com prises amagnetizable core 3, on which the same number of permanent magnets 4-4cas the number of pole shoes are mounted. The permanent magnets are somagnetized that the N and S poles face the pole shoes alternately andthat adjacent poles possess different polarity. The intermediate spaces5-50 between the permanent magnets should have a very high magneticresistance, i.e., they consist of non-magnetic material. The magnetizingsystem can be rotated independently of or in dependence upon therotation of the pole shoes around the common axis 311. The means for thepositioning of the magnetizing system and for the rotation thereofrelative to the pole shoes, and the driving means are conventional, sothey are not shown in FIGURE 1 and the following figures.

In FIGURE 1 there is a permanent magnet under each pole shoe. Sincethere is a high magnetic resistance between each two pole shoes in theform of the non-magnetic separating strip 2, the field lines closethrough the space outside the magnetic roller so that developer mixtureis attracted. For illustration, in FIGURE 1 the direction followed isshown by the field lines 6.

In FIGURE 2 the same magnetic roller is shown as in FIGURE 1 with thedifference that each permanent magnet is opposite a separating strip 2.The field lines 6a emerging at the north poles of the permanent magnets4 run inside the pole shoe 1 to the adjacent south poles so that thepole shoes do not produce a field outside the magnetic roller and thusdo not attract any developer mixture.

The change from the position of the magnetic system and pole shoes shownin FIGURE 1 to that shown in FIGURE 2 can be controlled automatically,e.g. by movement of the recording means carrying the latent image withconventional mechanical, hydraulic, pneumatic or electromagnetic meansbeing used to effect the necessary rotation. Preferably, e.g. in thecase of recording means in sheet form, during the developing process theposition shown in FIGURE 1 is maintained, whereas the renewal ofdeveloper between developing operations is effected by temporarilyadopting the position shown in FIGURE 2.

The magnetic roller shown in FIGURE 3 makes possible, in addition to therenewal of all of the attracted developer mixture on the roller byswitching off the magnetization, the continuous renewal of the developermixture during the developing process. This magnetic roller is thereforepracticularly suitable for the developing of latent electrostatic imageson record-carrying materials in the form of Webs of indefinite length.As in FIGURES 1 and 2, the magnetic roller of FIGURE 3 comprises softiron pole shoes 7, between which there are separating strips 8 ofnon-magnetic material. The permanent magnets 9 have rectangularcross-sections so they are simple and inexpensive to manufacture. Theintermediate spaces 10 are not magnetizable. The permanent magnets 9magnetized parallel to one edge of their cross-section are mounted on amagnetizable core 11 of polygonal cross-section in a manner such thatthe magnet poles point outwardly and, progressively around thecircumference of the roller, north and south poles alternate.

As in the case of the magnetic roller in FIGURES 1 and 2, in themagnetic roller shown in FIGURE 3, the magnetizing system comprising thecore 11 and the permanent magnets 9 are rotatable with respect to thepole shoes, so that during the developing process the permanent magnetscan be positioned opposite the pole shoes 7 and, for the purpose ofrenewing the developer, opposite the separating strips 8. On one part ofthe circumference there is incorporated, between the permanent magnets 9and the pole shoes 7, a magnetic short circuit in the form of acylindrically curved metal sheet 12 of highly permeable material whichdoes not rotate with the pole shoes and the magnetizing system but ismounted in fixed position. The

magnetic field lines 15a, which emerge from the magnet poles in the areaof the metal sheet 12, go through the metal sheet 12 to the adajcentpoles, so that the pole shoes in the area of the metal sheet 12 are notmagnetized.

Consequently, there occurs during the rotation of the magnetic roller aconstant renewal of the developer mixture held by the magnetic roller inthat the pole shoes, which are moved past the short circuiting metalsheet 12, let the developer mixture fall at 13 and take up new developermixture at 14 when they leave the area of the short circuiting metalsheet. In FIGURE 3, to indicate the field distribution in the area inwhich attraction of the developer mixture occurs, the magnetic fieldlines 15 are shown schematically.

When the magnetizing system comprises several permanent magnets ofsuitable dimensions which, in relation to the cores 3 or 11 in FIGURES lor 3, can be moved backwardly and forwardly individually in thedirection of rotation of the roller, it is also possible to keep aspatially fixed sector of the roller circumference constantlydemagnetized during operation of the magnetic roller without the shortcircuiting metal sheet 12. This can for example be elfected by bringingeach permanent magnet, when entering the sector, into position inrelation to the magnetic roller corresponding to the position shown inFIGURE 2 by moving each permanent magnet in a direction opposite to thedirection of rotation of the roller or in the direction of rotation.Each permanent magnet remains in the position shown in FIGURE 2 until inthe course of the rotation of the roller it leaves the sector when it ismoved back into the position corresponding to FIGURE 1. The necessarymovements can be effected easily with conventional mechanical devices.

The magnetizing system shown in FIGURES 1 and 3 can be simplified andthe cost thereof reduced by replacing every second permanent magnet by asoft magnetic body of the same shape.

In FIGURE 4 a further specific embodiment of the magnetizing system of amagnetic roller according to the invention is shown. It comprises amagnetizable core 16, over which is fitted a tube 17 of permanent magnetmaterial, magnetized in the radial direction, so there are an evennumber of magnet poles of alternating polarity uniformly distributedaround the circumference. The tube 17 may also be replaced by a suitablenumber of permanent magnet rings which are held together and are ofcorresponding magnetism.

FIGURE 5 shows another embodiment of the magnetizing system. In thiscase, the magnetizing system comprises a rod 18 of a permanent magnetmaterial of suitable length which is magnetized at the circumferencethereof in the manner shown. The magnetic field lines inside themagnetic rod run substantially in the direction of the circumference.The field lines 19 are shown for illustration.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. A magnetic roller for developing latent electrostatic imagescomprising a roller shell having a plurality of substantially axiallyparallel soft magnetic pole shoes on the circumference thereof, saidshoes being separated by non-magnetic strips having relatively highmagnetic resistance, and magnetizing means inside the roller shellincluding a plurality of substantially axially parallel permanent magnetmagnetic poles, the roller shell and the magnetizing means having acommon axis and being relatively rotatable to positions whereby amagnetic field of sufficient strength to hold magnetic developermaterial is produced on said roller and to positions where said magneticpoles are opposite said non-magnetic strips.

2. A magnetic roller according to claim 1 in which the number of poleshoes is even and equal to the number of magnetic poles.

3. A magnetic roller according to claim 1 in which the magnetic polesare carried by permanent bar magnets on the surface of a soft magneticcore.

4. A magnetic roller according to claim 3 in which the bar magnets aremovable relative to the core.

5. A magnetic roller according to claim 3 in which the number ofpermanent bar magnets is less than the number of. pole shoes.

6. A magnetic roller according to claim 1 in which the magnetic polesare carried by a magnetized permanent magnet hollow cylinder on a softmagnetic core.

7. A magnetic roller according to claim 1 in which the magnetic polesare carried by a cylinder magnetized substantially in a circumferentialdirection.

8. A magnetic roller according to claim 1 in which the pole shoes arecovered with a thin layer of non-magnetic material.

9. A magnetic roller according to claim 1 in which a highly permeablematerial is fixedly mounted between the roller shell and the magnetizingmeans over only a portion of the surface of the latter.

References Cited UNITED STATES PATENTS 2,177,809 10/1939 Queneau 209-2192,717,080 9/1955 Anderson 210222 2,786,440 3/1957 Giamio 11717.52,968,402 1/1961 Spodig 209-219 3,040,704 6/1962 Bliss 11717.5 3,098,7657/1963 Keller et al 117-17.5 3,152,924 10/1964 Wanielista et al 11717.5

FOREIGN PATENTS 507,101 5/1954 Canada.

191,-492 11/1907 Germany. 1,865,366 1/1963 Germany.

OTHER REFERENCES Indiana Steel Products Co., Indiana Permanent MagnetManual, No. 3, Permanent Magnets May do it Better,

WILLIAM D. MARTIN, Primary Examiner.

G. L. HUBBARD, E. J. CABIC, Assistant Examiners.

1. A MAGNETIC ROLLER FOR DEVELOPING LATENT ELECTROSTATIC IMAGES COMPRISING A ROLLER SHELL HAVING A PLURALITY OF SUBSTANTIALLY AXIALLY PARALLEL SOFT MAGNETIC POLE SHOES ON THE CIRCUMFERENCE THEREOF, SAID SHOES BEING SEPARATED BY NON-MAGNETIC STRIPS HAVING RELATIVELY HIGH MAGNETIC RESISTANCE, AND MAGNETIZING MEANS INSIDE THE ROLLER SHELL INCLUDING A PLURALITY OF SUBSTANTIALLY AXIALLY PARALLEL PERMANENT MAGNET MAGNETIC POLES, THE ROLLER SHELL AND THE MAGNETIZING MEANS HAVING A COMMON AXIS AND BEING RELATIVELY ROTATABLE TO POSTIONS WHEREBY A MAGNETIC FIELD OF SUFFICIENT STRENGHT TO HOLD MAGNETIC DEVELOPER MATERIAL IS PRODUCED ON SAID ROLLER AND TO POSITIONS WHERE SAID MAGNETIC POLES ARE OPPOSITE SAID NON-MAGNETIC STRIPS. 